1. Field of the Invention
The present invention generally relates to communication lines and, more specifically, to the protection of electronic equipment connected to telecommunication lines, be they telephone lines or digital data transmission lines, including of so-called private networks, of Ethernet type. The present invention more specifically relates to the protection of equipment connected to communication lines with no galvanic isolation transformer between the equipment and the line.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram illustrating an example of assembly of a circuit 1 for protecting (PROTECT) an equipment 2 (EQUIPMENT) connected to a telephone line (conductors TIP 4 and RING 5). The equipment may be any appliance, device, circuit, or installation connected to the line. For example, it may be a telephone set, a communication board (of modem type), a subscriber card of a telecommunication exchange or a distribution frame, etc. Circuit 1 has the purpose of carrying off the overvoltages arriving on the line so that they do not reach equipment 2 which is connected therewith and thus avoiding damaging it. The protection comprises short-circuiting the two line conductors 4 and 5 together.
Overvoltages may have various origins, especially lightning and incidental connections of the communication line to the electric supply system.
FIG. 2 shows a first conventional embodiment of protection circuit 1. Circuit 1 comprises two thyristors Th1 and Th2 in parallel and in reverse directions between the two conductors 4 (TIP) and 5 (RING). The gates of thyristors Th1 and Th2 are connected to their respective anodes by a zener diode DZ1 or DZ2. The anodes of thyristors Th1 and Th2 are respectively connected to conductors 4 (terminal A) and 5 (terminal B). When an overvoltage with a peak value greater than the threshold value of one of zener diodes DZ1 or DZ2 occurs on the line (between the two conductors), a current is injected into the gate of the thyristor with which this diode is associated. The thyristor triggers, which causes a short-circuit between the two conductors and cancels the overvoltage to protect equipment 2 downstream.
FIGS. 3A, 3B, 3C, and 3D illustrate, in timing diagrams, the operation of protection circuit 1 of FIG. 2 in the presence of a disturbance on the line. FIG. 3A shows an example of shape of current I1 arriving on one of the line conductors (for example 4). FIG. 3B shows the shape of current I2 from terminal A to equipment 2. FIG. 3C shows the shape of current I3 between terminals A and B of the protection circuit. FIG. 3D shows the shape of voltage V between terminals A and B.
In normal operation (before a time t0), current I1 present on the line is negligible as compared with that flowing in case of an overcharge and is thus not shown in the drawings. Circuit 1 is inactive and current I2 is equal to current I1. The occurrence of a disturbance at a time to, by an increase in current I1, is assumed. The occurrence of this disturbance causes an increase in voltage V between terminals A and B. When this voltage reaches triggering threshold VZ (FIG. 3D) of one of the zener diodes (in the example of orientation of the currents taken in the drawings, diode DZ1), a current (a portion of current I1) is injected into a gate of one of the thyristors (here, Th1) until said thyristor turns on (time t1). From this time t1, terminals A and B are short-circuited and current I3 in protection circuit 1 becomes equal to current I1 (neglecting the resistive losses in the on switches). Between times t0 and t1, the protection circuit is not turned on yet so that current I3 is null. Current I2 running to equipment 2 then follows the shape of voltage V.
A disadvantage is that with miniaturizations and the increasing integration of the electronic units forming communication equipment 2, the temporary overcharges (between times t0 and t1) before triggering of the protection may be sufficient to damage the components. Typically, current I2 during the turn-on phase can reach several amperes for a few microseconds. Such a power can be sufficient to damage the equipment to be protected.
FIG. 4 is a schematic block diagram illustrating a second example of a circuit 1′ for protecting a communication equipment 2. The principle is the same as in the first example, that is, establishing a short-circuit between conductors 4 and 5. As previously, two thyristors Th1 and Th2 are connected in parallel and in reverse directions between two terminals A and B respectively connected to conductors 4 and 5. The difference with the former assembly is the control of the thyristors which is performed by a circuit 11 (CTRL) measuring both the current between the two conductors and the current in one of them. In the example, the current is measured on conductor 4 by means of a current-to-voltage conversion resistor R11 interposed between terminal A and equipment 2. The terminals of resistor R11 are connected to circuit 11 which provides the signals for controlling the gates of thyristors Th1 and Th2. Circuit 11 generally draws its power supply from the line by having a terminal connected to terminal B (terminal on the conductor opposite to that having resistor R11) permanently connected between terminals A and B. This connection also enables measuring the voltage across the line to trigger the system in voltage mode as in the circuit of FIG. 2. When the current exceeds a threshold set by circuit 11, a control current is injected into one of the two gates according to the direction of the current, to turn on one of the two thyristors for, as in the case of FIG. 2, short-circuiting the line.
FIGS. 5A, 5B, and 5C illustrate, in timing diagrams, the operation of the protection circuit of FIG. 4. These drawings respectively show examples of shapes of currents I1 on conductor 4 upstream of the terminal A on the line side, 12 between terminal A and equipment 2, and 13 in circuit 1′. As previously, the coming of an overcharge is assumed at a time t0 from which current I1 increases, and current I2 follows the shape of current I1 as long as one of the thyristors is not turned on. Control circuit 11 triggers the turning-on from a current threshold IM. As for the previous circuit, the turning-on of one of the thyristors is not immediate so that current I2 keeps on increasing until this turning-on. This current may, as previously, reach several tens of amperes for a few microseconds and may thus be sufficient to damage the equipment to be protected.
In both cases, the risk is all the greater as the disturbance is abrupt (lightning).
According to whether the equipment to be protected is in a telecommunication system placed on the subscriber side or on the exchange side, the situation is such as illustrated where the line is formed of two conductors 4 (TIP) and 5 (RING) which are short-circuited, or a situation not shown for the time when each line to be protected is formed of a single conductor grounded by a protection circuit of the type of those illustrates in FIGS. 2 and 4.